DC Circuits Exam 3 Chapter 6

Wheatstone Bridge

A Wheatstone bridge is used to precisely measure resistance A Wheatstone bridge is also applied with transducer measurements, to measure physical quantities such as temperature, strain, and pressure, where small transducer resistance changes may need to be precisely measured Tiny changes in transducer resistance will unbalance the bridge, thereby providing a measurement reading

Unloaded Voltage Dividers

A voltage divider produces an output which depends upon the values of the dividing resistors This voltage is the unloaded output voltage

Load Current and Bleeder Current

Bleeder current is the current left (I3) after the total load current is subtracted from the total current into the circuit

Maximum Power Transfer

Maximum power is transferred from a source to a load when the load resistance is equal to the internal source resistance The source resistance, RS, of a circuit is the equivalent resistance as viewed from the output terminals using Thevenin's theorem A typical application of the maximum power transfer theorem is in audio systems, where the speaker resistance must be matched to the audio power amplifier in order to obtain maximum output

Summary of Thevenin's Theorem

Open the two terminals (remove any load) between which you want to find the Thevenin equivalent circuit Determine the voltage (VTH) across the two open terminals Determine the resistance (RTH) between the two open terminals with all sources replaced with their internal resistances (short voltage sources and open current sources) Connect VTH and RTH in series to produce the complete Thevenin equivalent for the original circuit Place the load resistor removed in Step 1 across the terminals of the Thevenin equivalent circuit. The load current and load voltage can now be calculated using only Ohm's law. They have the same value as the load current and load voltage in the original circuit

Superposition Theorem

Some circuits require more than one voltage or current source The superposition theorem is a way to determine currents and voltages in a circuit that has multiple sources by considering one source at a time

Applying Superposition Theorem

Take one voltage (or current) source at a time and replace the remaining voltage sources with shorts (and remaining current sources with opens) Determine the particular current or voltage that you want, just as if there were only one source in the circuit Take the next source in the circuit and repeat Steps 1 and 2 for each source To find the actual current in a given branch, algebraically sum the currents due to each individual source. Once the current is found, voltage can be determined by Ohm's law

Balanced Wheatstone Bridge

The Wheatstone bridge is in the balanced bridge condition when the output voltage between terminals A and B is equal to zero

General statement of Superposition Theorem

The current in any given branch of a multiple-source circuit can be found by determining the currents in that particular branch produced by each source acting alone, with all other sources replaced by their internal resistances. The total current in the branch is the algebraic sum of the individual source currents in that branch

What Happens when a Load Resistor is connected across a voltage-divider output?

The output voltage decreases

Unbalanced Wheatstone Bridge

The unbalanced bridge, when VOUT is not equal to zero, is used to measure some transducer quantities, such as strain, temperature, or pressure The bridge is balanced at a known point, then the amount of deviation, as indicated by the output voltage, indicates the amount of change in the parameter being measured

Thevenin's Theorem

Thevenin's theorem provides a method for simplifying a circuit to a standard equivalent form The Thevenin equivalent voltage (VTH) is the open circuit (no-load) voltage between two terminals in a circuit The Thevenin equivalent resistance (RTH) is the total resistance appearing between two terminals in a given circuit with all sources replaced by their internal resistances

A balanced Wheatstone bridge is used for?

To measure an unknown resistance

Total Current

Using the total resistance and the source voltage, find the total current by applying Ohm's law IT = VS/RT

Loaded V D

When a load resistor RL is connected from the output toa ground, the output voltage is reduced by an amount that depends on the value of RL

Loading Effect of a Voltmeter

When measuring across a resistor, a voltmeter is connected in parallel with the resistor Being in parallel, the internal resistance of the voltmeter will have a loading effect on the circuit that is being measured Modern digital voltmeters (DMM) have an internal resistance of 10Mohms If the meter resistance is at least ten times greater than the resistance across which it is connected, the loading effect can be neglected measurement error is less than 10%

Any two-terminal resistive circuit?

can be replaced by its Thevenin equivalent, made up of an equivalent resistance (RTH) in series with an equivalent voltage source (VTH)

unbalanced Wheatstone bridge can be used

to measure physical quantities using transducers